The CXBus network provides the information necessary to facilitate the exchange of data and control information over the coax medium within a home. The detailed description of the CXBus network may be found in the "CX Specification, IS-60.3 Coax Physical Layer and Media Specifications", revised Mar. 19, 1992, which is incorporated herein by reference for its teachings on CXBus topologies, media and devices, including Node "0" definition. CXBus is also described in O'Brien Jr., "Physical and media specifications of the CXBus", IEEE Transactions on Consumer Electronics, Vol. 37, Number 3, August, 1991, which is incorporated by reference herein for its teachings on CXBus networks. A publication by J. Hofmann entitled "The Consumer Electronic Bus: an Integrated Multi-Media LAN for the Home", International Journal of Digital and Analog Communication Systems, Vol. 4, 77-86, (1991), (which is incorporated herein by reference for its teachings on CEBus network architectures) describes issues related to frequency placement of block converted signals.
The Consumer Electronic Bus (CEBus) is a multimedia LAN standard for use by the Electronics Industry Association's Consumer Electronics Bus Committee. CEBus provides a standard communications interface to each of six different media. These media include: PLBus (Power Line Bus), TPBus (Twisted-Pair Bus), CXBus (Coaxial Bus), IRBus (Infrared Bus), RFBus (Radio Frequency Bus) and FOBus (Fiber-Optic Bus). CEBus comprises media with data channels and a control channel. The control channel carries commands used to allocate the data channels, as well as short messages. The data channels carry a variety of signals which do not fit within the bandwidth constraints of the control channel. The control channel is digital, whereas the data channels may carry signals of any type.
CXBus network configurations support all anticipated coax connected appliances in the home. The CXBus distribution system includes a collection of coaxial cable pairs that originate at a special node distribution device called Node "0" 800, as shown in FIG. 8. This dual cable approach provides sufficient spectrum space for externally generated signals from either a Cable Television (CATV) cable or Off-Air (i.e., from an outside antenna), as well as the video and audio signals generated within the home (e.g., from a video cassette recorders or surveillance cameras). The two cables of this cable pair are described as "external" cable and "internal" cable respectively.
The CXBuS network may be a single independent network. The CXBus network may also be interconnected with other CXBus networks or different CEBus media by using a Router, Bridge or Brouter.
FIG. 10 is a block diagram of a conventional Node "0" 800. The CXBus network consists of one or more coaxial cable pairs attached at Node "0" 800 and terminated by a 4-way splitter/combiner 822. The Node "0" 800 of FIG. 10 includes circuitry to provide the following functions:
1. An input function 802 for combining signals from Internal cable branches and for distributing signals to the internal cable branches.
2. A block converter 804 for block frequency conversion of data channel signals 806 for distribution back onto the Internal cable (not shown) via input function 802 or External cable (not shown) via external cable output function 808.
3. An amplifier 812 that amplifies the block converted data channel signals 810 to drive the required number of External cable or Internal cable branches, overcoming the losses of splitters and cables to deliver the required signal strength to the distribution device.
4. A CATV/Off-Air function 814 that provides the filtering and amplification needed for distributing CATV or Off-Air television signals.
5. A control channel regenerator 816 that receives the 5.5 MHz signal, demodulates the signal, modulates a control channel signal on a 4.5 MHz channel, and provides the 4.5 MHz control channel at sufficient signal strength on all Internal cables.
6. A Node "0" device may also provide a CATV signal return path for two-way cable services as shown in FIG. 10.
High quality audio/video distribution is one of the important features of the CXBus network. The quality of the distributed video signal in the CXBus network is heavily influenced by the following five factors:
1. The Signal/Noise (S/N) ratio of the video source which is used in the network;
2. the Carrier/Noise (C/N) ratio of the output signal from any modulator which generates signals to be distributed on the CXBus;
3. the signal level that is allowed to be placed on the network;
4. the topology of the CXBus network; and
5. the noise figure of the Node "0".
The C/N ratio (RF video carrier to noise power ratio) is one of the major parameters considered when evaluating the quality of CATV distribution system. This noise component depends on the quality of the modulators used for the internal sources. The C/N ratio may be reduced by changing the signal source. There is one noise component, however, that cannot be reduced. The irreducible noise is the thermal noise. For a TV channel signal with an ideal 75 ohm impedance and 4 MHz bandwidth, the irreducible noise level is about -59 dBmV at room temperature 70.degree. F.
For the CXBus network, the in-home generated audio/video signals are block converted by Node "0" 800, and the block converted signals may be fed back from the internal cable or external cable.
In most current cable systems, channels are allocated below 450 MHz, leaving the High band between 450 and 546 MHz open for carrying block converted signals over the external cable. This allows internally generated signals to be rebroadcast by Node "0" over the external cable. Devices throughout the CXBus network may receive signals on the external cable from either the CATV or from the internal signal source (e.g., VCR without switching the devices from the external cable to the internal cable.
The inventors have determined that, if the nominal signal level S.sub.rec received at the device (e.g. TV) is +4 dBmV, the required output signal level from the block converter S.sub.BLOCK (considering only 4 coaxial cable pairs being driven by this Node "0" 800) is +29 dBmV.
When the block converted signals 810 are fed back to the internal cable input section 802, this +29 dBmV signal leaks into the input of the block converter 804 through the 2-way splitter/combiner 818 and becomes extra noise relative to the signal 806 coming from the device sources (e.g., a Video Cassette Recorder, VCR). It is necessary to add an additional band pass filter 820 in front of the block converter 804, but this causes additional signal loss, lowering the C/N ratio.
Because the passive splitter/combiner 822 is used at the input 802 of the Node "0" 800, the signal level 806 decreases when the number of coaxial cable pairs attached at Node "0" increases. This means that the quality of the distributed video signal 824 decreases rapidly if the size of the CXBus network expands.
Based on the above design considerations, if the block converter 804 has a 4.0 dB noise figure, and only 4 coaxial cable pairs are attached at Node "0" 800, then the maximum achievable C/N ratio at the output port of the block converter 804 is worse than 36 dB even with a perfect noiseless video source and TV modulator. The relation between the C/N ratio (without modulation) and the picture quality is shown in Table 1.
TABLE 1 ______________________________________ C/N Ratio (dB) Picture Quality ______________________________________ over 42 dB Excellent 40 dB-42 dB Good-Excellent 38 dB-40 dB Fair-Good 36 dB-38 dB Passable-Fair 34 dB-36 dB Objectionable-Passable under 34 dB Unusable ______________________________________
Based on table 1, a Node "0" 800 having a conventional passive input section is not practical for distributing a video signal through the CXBus network. The quality of the picture from the video signals provided by Node "0" 800 is marginal at best.
In order to provide a CXBus network that is able to distribute high quality audio/video signals, a very low noise figure for Node "0" is desired.